In a field of optical measurement or optical communication, optical waveguide devices such as an optical modulator that uses a substrate having an electro-optical effect are widely used. In addition, so as to realize a wider bandwidth of frequency response characteristics or so as to reduce a drive voltage, the substrate is made to have a small thickness of approximately 10 μm, whereby an effective refractive index of a microwave as a modulation signal is lowered, speed matching between the microwave and an optical wave is contrived, and improvement in electric field efficiency is contrived. As shown in PTL 1, this thin substrate is used after being adhered to another supporting substrate to have mechanical strength.
In a thin main substrate (also, referred to as a “thin substrate”), the behavior of signal light that propagates through the inside of the substrate becomes problematic. Particularly, stray light that leaks from an optical waveguide inside a thin substrate, for example, an S-shaped portion and a branching and coupling portion propagates through a thin substrate as a slab waveguide differently from a case in which the main substrate has thickness of 100 μm or more. In addition, when slab propagation light, which is reflected from an end face of the optical waveguide device, intersects the optical waveguide at a small angle, optical coupling occurs, and thus wavelength dependency of the optical waveguide device occurs in accordance with a coupling state. Furthermore, wavelength dependency occurs in propagation loss or a phase difference of a monitoring photo detector that monitors an extinction ratio, and thus an operational characteristic of the optical waveguide device deteriorates.
Specifications of the loss or extinction ratio are defined by the wavelength bands that are used, and thus it is necessary to measure the characteristic in all of the wavelength bands that are used. As a result, the measurement takes a long time, and when the characteristic is inferior even in one wavelength, this has a significant effect on a yield rate.